There are a wide variety of light sources in existence, e.g., incandescent lights, fluorescent lamps, solid state light emitters, laser diodes, thin film electroluminescent devices, light emitting polymers (LEPs), halogen lamps, high intensity discharge lamps, electron-stimulated luminescence lamps, etc. The various types of light sources have been provided in a variety of shapes, sizes and arrangements, e.g., A lamps, B-10 lamps, BR lamps, C-7 lamps, C-15 lamps, ER lamps, F lamps, G lamps, K lamps, MB lamps, MR lamps, PAR lamps, PS lamps, R lamps, S lamps, S-11 lamps, T lamps, Linestra 2-base lamps, AR lamps, ED lamps, E lamps, BT lamps, Linear fluorescent lamps, U-shape fluorescent lamps, circline fluorescent lamps, single twin tube compact fluorescent lamps, double twin tube compact fluorescent lamps, triple twin tube compact fluorescent lamps, A-line compact fluorescent lamps, screw twist compact fluorescent lamps, globe screw base compact fluorescent lamps, reflector screw base compact fluorescent lamps, etc. The various types of light sources have been supplied with energy in various ways, e.g., with an Edison connector, a battery connection, a GU-24 connector, direct wiring to a branch circuit, etc. The various types of light sources have been designed so as to serve any of a variety of functions (e.g., as a flood light, as a spotlight, as a downlight, etc.), and have been used in residential, commercial or other applications.
With many light sources, there is a desire to effectively dissipate heat produced in generating light.
For example, with many incandescent light sources, about ninety percent of the electricity consumed is released as heat rather than light. There are many situations where effective heat dissipation is needed or desired for such incandescent light sources.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. A challenge with solid state light emitters is that many solid state light emitters do not operate as well as possible when they are subjected to elevated temperatures. For example, many light emitting diode light sources have average operating lifetimes of decades (as opposed to just months or 1-2 years for many incandescent bulbs), but some light emitting diodes' lifetimes can be significantly shortened if they are operated at elevated temperatures. A common manufacturer recommendation is that the junction temperature of a light emitting diode should not exceed 70 degrees C. if a long lifetime is desired.
In addition, the intensity of light emitted from some solid state light emitters can vary based on ambient temperature. For example, light emitting diodes that emit red light often have a very strong temperature dependence (e.g., AlInGaP light emitting diodes can reduce in optical output by ˜20% when heated up by ˜40 degrees C., that is, approximately −0.5% per degree C.; and blue InGaN+YAG:Ce light emitting diodes can reduce by about −0.15%/degree C.). In many lighting devices that include solid state light emitters as light sources (e.g., general illumination devices that emit white light in which the light sources consist of light emitting diodes), a plurality of solid state light emitters are provided that emit light of different colors which, when mixed, are perceived as the desired color for the output light (e.g., white or near-white). The desire to maintain a relatively stable color of light output is therefore an important reason to try to reduce temperature variation of solid state light emitters.
In some cases (e.g., most residential applications), fixtures (e.g., “cans”) are required to be substantially airtight around the sides and top to prevent the loss of ambient heat or cooling from the room into the ceiling cavity through the fixture. As the lamp is mounted in the can, much of the heat generated by the light source is trapped within the can, because the air heated in the can rises and is trapped within the can. Insulation is usually required around the can within the ceiling cavity to further reduce heat loss or cooling loss from the room into the ceiling cavity.
General illumination devices are typically rated in terms of their color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of an illumination system compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the illumination system are the same as the coordinates of the same test colors being irradiated by the reference radiator.
Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra).
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light.
“White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps (referred to as “RGB lamps”) use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light. While there is a need for more efficient white lighting, there is in general a need for more efficient lighting in all hues.